This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Post-translational modifications of proteins are critical for regulating cellular processes and our understanding of these processes relies on the ability to determine the precise location of these modifications. Tandem mass spectrometry is potentially well suited for the determination of post-translational modification sites. Several methods have been proposed to determine the most likely location for a modification given a tandem mass spectrum, but currently there is no framework for judging a localization assignment in terms of a statistical significance level [unreadable]i.e., the risk that a localization assignment has occurred by chance.